1. Field of the Invention
The present invention relates to an automatic shift transmission for vehicles. Particularly, it relates to an automatic shift transmission for vehicles of the type in which the shifting of speed is effected with a changeover valve operable with a pilot type solenoid valve connected to a vent line.
2. Description of Relevant Art
Conventionally, in automatic shift transmissions of the type in which the shifting of speed is effected with at least one changeover valve adapted for the selective supply of hydraulic oil to a plurality of speed shift clutches and in which, as directly operated with the solenoid valve, the changeover valve for controlling the selection of gear ratio needs a relatively large electric current to be conducted through the solenoid valve, thus increasing the heat liberation, there has been employed a relatively small solenoid valve of a poppet type as a pilot valve for the changeover valve.
For operating a changeover valve of a spool type with such a poppet type pilot valve, there have been proposed automatic shift transmissions for vehicles in which the spool valve is normally biassed at its either end with a spring so as to be held at its first changeover position and supplied at the other end with a hydraulic pressure introduced thereto from a hydraulic shift line through an orifice so as to move against the resilient force of the spring to its second changeover position and in which, by opening and closing the poppet valve installed downstream of the orifice, the changeover valve is changed over between the first and second changeover positions.
In those conventional automatic shift transmissions, however, upstream of the orifice there is always existing a line pressure, which may be extremely raised with a high load, thus causing the pilot valve as opened to vent throught the orifice a considerable amount of hydraulic oil, with the result of poor economy.
Moreover, even in a cruising in which the line pressure is controlled at a constant level without being raised, the pressure for changing over the spool valve is not needed to be so high as the line pressure and, therefore, also with respect to the amount of hydraulic oil vented through the orifice in this case, it has been an object for automatic shift transmissions to possibly reduce the oil consumption.
The present invention has been achieved to effectively solve such problem in the conventional automatic shift transmissions for vehicles.